Boris Johnson was accused of “macho nonsense” and "ridiculous willy-waving" over his refusal to meet union boss Bob Crow as talks over the second round of 48-hour Tube strikes, due to take place this week, remain finely balanced.

In a heated row over the future of the Underground, a senior London Labour MP criticised the Mayor for “ridiculous willy-waving” for leaving negotiations to Transport for London officials rather then meeting face-to-face.

Both sides resumed talks at ACAS today with efforts focused on agreeing a form of words that would allow a consultation to be extended and strike action suspended.

But, reacting to suggestions that talks were increasingly constructive, a source close to negotiations said any progress was prone to being quickly derailed, and while both sides were keen to appear conciliatory in public they acted differently behind closed doors.

Islington South and Finsbury MP Emily Thornberry challenged the Mayor on the BBC’s Sunday Politics London show, saying: “How mad is it that you haven’t spoken to [Bob Crow] for five years? He has to call you up on LBC to talk to you. It’s not right.

“It’s nonsense why the leader of London is not talking to the leader of the Underground union.

“It’s just the most ridiculous bit of willy-waving I’ve seen. It’s just macho nonsense not to talk to the leader of the union.”

But Mr Johnson defended himself from the “sexist assault” insisting: “We’ve been the reverse of macho. We have engaged with the unions at every stage ... this change is going to happen.”

The second wave of 48-hour strike action is set to begin tomorrow at 9pm with services not fully restored until Friday morning.